Chandra has a busy observing schedule

Chandra has busy observing schedule

Dying magnetars, supernovae, and
the future of the universe are on tap for NASA's next Great Observatory

July 20, 1999: Dying magnetars,
supernovae in multicolor coats, everyone's favorite Crab, and
the future of the Universe are on the observing schedule for
scientists at NASA's Marshall Space Flight Center when the Chandra
X-ray Observatory starts observing the heavens later this year.

While NASA/ Marshall managed the Chandra project since the
mid-1970s, and played an active role in calibrating its telescope,
it also has been preparing to use Chandra as a science tool.
Launch of Chandra is now set for Thursday, July 22. Within a
few months of launch, Chandra will gradually emerge from verification
tests and be turned into an operational observatory. Among the
scientists waiting to use it as guest investigators are four
at NASA/Marshall.

Most objects in the sky can be pigeonholed into a few of the
hundreds of categories that classify stars, galaxies, and other
bodies. Every now and then, you get one that changes its colors
- literally - and seems to beg for closer examination. That's
the case with a supernova known as SN 1993J.

"It started out
as a classic Type II supernova," said Dr. Douglas Swartz,
"with hydrogen lines in its spectrum. These weakened in
a few weeks and helium lines appeared, more like a Type Ib supernova."

In effect, SN 1993J changed its appearance from a supernova
caused by an ordinary massive star to a supernova caused by a
dense helium stellar core.

Left: A radio telescope view of SN 1993J expanding.

To help resolve the apparent conflict, Swartz will use Chandra,
which he also helped calibrate.

Because the supernovae have different origins, they emit light
differently as they explode. But SN 1993J is a transition object
which had lost most, but not all, of its hydrogen envelope.

"Supernova 1993J is one of the few that has made the
transition from one type to another," Swartz explained.
Only one other supernova, SN 1987K, has been seen making such
a change. With Chandra, Swartz hopes to see X-ray signatures
of the chemical makeup of the outrushing material, including
the hydrogen that disappeared a few weeks after the blast.

In early summer of 1054, long before the first Independence
Day celebration in the United States, the people of Japan and
China witnessed an amazing display of fireworks in the summer
sky. The Crab Nebula, as the display came to be known, was an
exploding new supernova so bright it was visible in the daytime
sky for nearly a month. Soon after, it faded to a level where
it would not be rediscovered until newly invented telescopes
spotted it in the 18th century.

"The Crab Nebula
and the star at the center of it are the Rosetta Stone of modern
astrophysics," said Dr. Martin Weisskopf, the Chandra project
scientist. The neutron star at the center is known as the Crab
Pulsar. It is classified as a pulsar because of its flashing
nature - it sends bursts of energy out 33 times a second with
reliability rivaling that of our most dependable clocks.

Aside from being the most observed of all pulsars, the Crab
Pulsar is also believed to be the youngest of more than 700 known
to astronomers.

Left: A view of the Crab Nebula, including a detailed
view of the core as seen by the Hubble Space Telescope.

"Since it is the youngest, it's also the hottest,"
explained Weisskopf, "and X-rays offer the best way to observe
it at these temperatures. Neutron stars are a unique laboratory
for probing various physical phenomena. Of interest here is the
thermal evolution of the stars." The physical activity in
the star's superfluid interior, under a crystalline neutron crust,
is impossible to recreate in any laboratory on Earth, so scientists
have been working up theories based on observations of the Crab
Pulsar and other neutron stars.

The high resolution camera aboard Chandra will help Weisskopf
and other scientists test the theories by giving them a better
reading of the temperature on the surface of the Crab Pulsar.

"The more resolution, the better," said Weisskopf.
"Right now we're looking at the glow of activity near the
center of the nebula as you might see the glow of city lights
from a distance. Examining the pulsar in the center using Chandra
will be like using a telescope to focus on a single street light
in the middle of the city."

Looking for pulsars living in the fast
lane

The discovery in 1998 of the first magnetar - a highly magnetized
star - also put the spotlight on a small class of stars called
Anomalous X-ray Pulsars, or AXPs. While the magnetar discovery
involved Soft Gamma Repeaters (SGR), the magnetar theory holds
that these objects become AXPs before they fade from the scene
altogether.

Dr. Jan van Paradijs, who won the 1997 Rossi Prize for identifying
the first optical counterpart for a gamma ray burster, hopes
to use Chandra to take a closer look at two AXPs - one third
of the known population.

"The reason I'm interested in them is that
I suspect they're magnetars," said van Paradijs, an astronomer
with the University of Amsterdam and the University of Alabama
in Huntsville and working at NASA/Marshall.

Right: An scientist's concept of a magnetar surrounded
by hot plasma. The blue lines represent magnetic field lines.
credit: Robert Mallozzi/University of Alabama at Huntsville and
NASA/MSFC

Short life is the price of being a magnetar. The SGR phase
lasts only 10,000 years, and may be followed by the AXP phase
for another 10,000 years or so. Until recently, astronomers did
not link the SGR and AXP classes. Indeed, the AXP class was formed
because a handful of X-ray pulsars did not fit into the normal
categories.

"It's a combination of things," van Paradijs said.
"They have an awkward spectrum and they also have a very
limited period range which says there's something very special
going on." Some are associated with relatively recent supernova
remnants. All these factors, van Paradijs believes, add up to
a strong magnetic field that is aging the pulsar faster than
normal.

"With Chandra, we hope to get evidence that they are
indeed magnetars," van Paradijs said. Unfortunately, it
is not likely that he will be able to hunt for what comes after
the AXP phase. After 20,000 years, magnetars are believed to
fade away from notice.

Measuring the scale of the Universe

Combining measurements by Chandra with radio telescope observations
may help refine our understanding of the age of the universe.

"We could be on the brink of answering some important
cosmological questions," says Dr. Marshall Joy, "but
we need more data that only Chandra can provide."

Joy and Dr. John Carlstrom, of the University of Chicago,
plan to measure the age of the Universe with the little-known
Sunyaev-Zeldovich Effect. This happens when microwaves from the
cosmic microwave background radiation collide with hot gas between
distant galaxies and are scattered at higher energies. The effect
is a slight dip in radio intensity in an otherwise smooth background.

"Chandra has the collecting area and the sensitivity
to high energy X-rays to make high quality images of the hot
cluster atmospheres. If we could get radio maps of the same clusters
and measure the size of the Sunyaev-Zeldovich effect, we could
begin to do some interesting cosmology."

Joy and Carlstrom plan to combine X-ray data from Chandra
with their existing radio images to estimate the absolute distances
to a number of galaxy clusters. By
combining their distance measurements with optical redshift data
they can calculate the Hubble constant for each cluster.

Right: Radio telescope and early X-ray views of an Abell
cluster of galaxies one of the observing targets for S-Z effect
measurements.

"We can also use Sunyaev-Zeldovich measurements to draw
some conclusions about the total mass in the Universe,"
said Carlstrom. From that can come a better estimate of whether
the universe will expand forever, or eventually slow and collapse
on itself.

"We're optimistic", said Carlstrom, "because we'll
be getting real answers to some of the most important questions
in cosmology."